The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It will be appreciated, however, that the invention will also have other applications.
Typically, a high power x-ray tube includes an evacuated envelope or housing which holds cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and an anode which is also located in the evacuated envelope. This potential causes the electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area of the anode or focal spot with sufficient energy that x-rays are generated and extreme heat is produced as a byproduct.
In high energy x-ray tubes, the anode is rotated at a high speed such that the electron beam does not dwell on only the small spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam. Larger diameter anodes have larger circumferences, hence provide greater thermal loading. In conventional rotating anode x-ray tubes, the envelope and the cathode remain stationary while the anode Forages inside the envelope. Heat from the anode is dissipated by the thermal radiation through the vacuum to the exterior the vacuum envelope. It is to be appreciated that heat transfer from the anode through the vacuum is limited.
High power x-ray tubes have been proposed in which the anode and vacuum envelope rotate, while the cathode filament inside the envelope remains stationary. This configuration permits a coolant fluid circulate in directed contact with the anode to provide a direct thermal communication between the anode and the exterior of the envelope. See for example, U.S. Pat. Nos. 5,046,186: 4,788,705; 4,878,235; and 2,111,412.
More specifically, an outer housing is provided which has the window through which x-rays emerge. The anode and vacuum envelope are rotatably mounted within the housing and displaced a significant distance therefrom. The chamber between the housing and the vacuum envelope is filled with a coolant oil. Connections are provided on the housing for withdrawing the oil, pumping it through a radiator or other cooling system, and returning the cooled oil to the housing. When x-rays are generated at the focal point on the anode, x-rays are emitted in substantially all directions. Because the anode has a high x-ray blocking power, x-rays are effectively emitted over a hemispherical volume defined over the focal point where the electron beam from the cathode strikes the anode surface. These high energy x-rays pass through the vacuum envelope into the coolant oil. The coolant oil is highly radiation transparent such that x-rays passes through the oil in the reservoir to the window without significant attenuation.
One of the difficulties with this configuration is that x-ray tube oil that has good thermal characteristics does not necessarily have a high x-ray tolerance. Quite to the contrary, many high temperature cooling oils are degraded badly by x-rays.
Another difficulty with this configuration is that the focal point where the electron beam is focused on the anode to cause the generation of x-rays is not truly a point. Rather, x-rays are generated over an area of some physical size. In CT scanners and other precision x-ray equipment, the x-rays from other than the theoretical point source, known as off-focal radiation, tend to degrade the resultant image. To limit off-focal radiation, collimators are commonly installed as a part of the housing window. Although collimators are and can be provided in other portions of the CT scanner, off-focal radiation collimation in the window is preferred because it has been found that collimating for off-focal radiation as close to the x-ray source as possible is the most effective.
Another difficulty with this configuration is that there is significant frictional drag between the rotating vacuum envelope and the coolant oil. An order to run the high powered x-ray tubes on a continuous duty cycle, the anode is typically run at a relatively nigh velocity. A relatively large horsepower motor is required to overcome the frictional drag of the coolant fluid on the rotating envelope.
The present invention provides a new and improved construction which overcomes the above-referenced problems and others.